Asymmetric total synthesis of (+)-danicalipin A

Manganese-Catalyzed 5-Endo-trig Oxygenative Cyclization of α,β-Unsaturated Oximes under Air and Ambient Conditions for the Synthesis of 4,5-Dihydroisoxazoles

The stereoselective 5-endo-trig oxygenative cyclization of α,β-unsaturated oximes was achieved using molecular oxygen (O2) and a manganese catalyst. Several 4-hydroxy-4,5-dihydroisoxazoles were obtained in high yields by directly incorporating O2 from the atmosphere (eliminating the necessity for a pure oxygen environment) and using an unprecedentedly low loading of Mn(acac)3 (as little as 0.020 mol %) without additional additives. Because of its desirable features, such as operational simplicity, inexpensive catalyst, mild reaction conditions (open flask conditions at room temperature), and broad substrate compatibility, this novel reaction provides an attractive synthetic approach to producing 4-hydroxy-4,5-dihydroisoxazoles.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

Three-component addition of terminal alkynes, carboxylic acids, and tert-butyl hypochlorite

This paper described the unprecedented three-component addition of terminal alkynes, carboxylic acids, and tert-butyl hypochlorite. This new type of addition proceeds smoothly to produce gem-dichloroalkane derivatives in satisfactory to excellent yields via successive two-time additions under mild conditions. The synthetically useful functional groups, such as Cl, Br, CN, and NO₂, remained intact in the final products.

Fundamental curiosity of multivicinal inter-halide stereocenters

A stereoselective strategy allowed the striking impact of a single halogen on the physical properties of inter-halide alkane units to be unravelled.

Saegusa Oxidation of Enol Ethers at Extremely Low Pd-Catalyst Loadings under Ligand-free and Aqueous Conditions: Insight into the Pd(II)/Cu(II)-Catalyst System

A highly efficient and practical Pd(II)/Cu(OAc)₂-catalyst system of Saegusa oxidation, which converts enol ethers to the corresponding enals with a number of diverse substrates at extremely low catalyst loadings (500 mol ppm) under ligand-free and aqueous conditions, is described. Its synthetic utility was demonstrated by large-scale applications of the catalyst system to important nature molecules. This work allows Saegusa oxidation to become a highly practical approach to preparing enals and also suggests new insight into the Pd(II)/Cu(II)-catalyst system for dehydrogenation of carbonyl compounds and decreasing Pd-catalyst loadings.

Neighboring Effect of Intramolecular Chlorine Atoms on Epoxide Opening Reaction by Chloride Anions

We investigated the diastereoselectivity of ring openings for chloro vinyl epoxides with various chlorination reagents. In the chlorinolysis reactions using vinyl epoxides having an allyl alcohol, inversion:retention ratios varied depending on the chloride sources. In limited cases, the increase in retention ratio was consistent with the intervention of chloronium ions. In contrast, all vinyl epoxides bearing an α,β-unsaturated ester gave only the inversion products. These results suggest the electron-withdrawing property suppressed the chloronium ions.

Catalytic Enantioselective Dihalogenation in Total Synthesis

To date, more than 5000 biogenic halogenated molecules have been characterized. This number continues to increase as chemists explore chloride- and bromide-rich marine environments in search of novel bioactive natural products. Naturally occurring organohalogens span nearly all biosynthetic structural classes, exhibit a range of unique biological activities, and have been the subject of numerous investigations. Despite the abundance of and interest in halogenated molecules, enantioselective methods capable of forging carbon-halogen bonds in synthetically relevant contexts remain scarce. Accordingly, syntheses of organohalogens often rely on multistep functional group interconversions to establish carbon-halogen stereocenters. Our group has developed an enantioselective dihalogenation reaction and utilized it in the only reported examples of catalytic enantioselective halogenation in natural product synthesis. In this Account, we describe our laboratory's development of a method for catalytic, enantioselective dihalogenation and the application of this method to the synthesis of both mono- and polyhalogenated natural products. In the first part, we describe the initial discovery of a TADDOL-mediated dibromination of cinnamyl alcohols. Extension of this reaction to a second-generation system capable of selective bromochlorination, dichlorination, and dibromination is then detailed. This system is capable of controlling the enantioselectivity of dihalide formation, chemoselectivity for polyolefinic substrates, and regioselectivity in the case of bromochlorination. The ability of this method to exert control over regioselectivity of halide delivery permits selective halogenation of electronically nonbiased olefins required for total synthesis. In the second part, we demonstrate how the described dihalogenation has provided efficient access to a host of structurally diverse natural products. The most direct application of this methodology is in the synthesis of naturally occurring vicinal dihalides. Chiral vicinal bromochlorides represent a class of >175 natural products; syntheses of five members of this class, including its flagship member, (+)-halomon, have been accomplished through use of the catalytic, enantioselective bromochlorination. Likewise, enantioselective dichlorination has provided selective access to two members of the chlorosulfolipids, a class of linear, acyclic polychlorides. Synthesis of chiral monohalides has been achieved through solvolysis of enantioenriched bromochlorides; this approach has resulted in the synthesis of five bromocyclohexane-containing natural products through an enantiospecific bromopolyene cyclization. In reviewing these syntheses, a framework for the synthesis of chiral organohalogens mediated by catalytic, enantioselective dihalogenation has emerged. The development of a selective dihalogenation method has been highly enabling in the synthesis of halogenated natural products. In this Account, we detail all examples of catalytic, enantioselective halogenation in total synthesis and encourage the further development of synthetically useful halogenation methodologies.

Synthesis of Vicinal Dichlorides via Activation of Aliphatic Terminal Epoxides with Triphosgene and Pyridine

Herein we report a novel synthetic reaction to convert unactivated terminal aliphatic epoxide to alkyl vicinal dichloride based on triphosgene-pyridine activation. Our methodology is operationally simple and readily tolerated by a broad of scope of substrates as well as protecting groups. Furthermore, these mild conditions generally yield clean reaction mixtures that are free of byproducts upon aqueous workup.

Stereochemistry and biological activity of chlorinated lipids: a study of danicalipin A and selected diastereomers

The syntheses of (+)-16-epi- and (+)-11,15-di-epi-danicalipin A (2 and 3) are reported. The conformations of the parent diols 5 and 6 as well as the corresponding disulfates 2 and 3 were determined on the basis of J-based configuration analysis and supported by calculations. The impact of configuration on membrane permeability in Gram-negative bacteria and mammalian cell lines was assessed as well as cytotoxicity. Although diastereomer 2 showed similar behavior to natural (+)-danicalipin A (1), strikingly, the more flexible C11,C15-epimer 3 had no effect on permeability and proved equally or more toxic towards multiple cell lines.

Enantioselective Addition of Alkynes to α,α-Dichlorinated Aldehydes

Enantioselective addition of terminal alkynes to α,α-dichlorinated aldehydes employing Zn(OTf)₂/NME is disclosed. The propargylic alcohols obtained are accessed in good yields and high enantioselectivity from easily accessible α,α-dichloroaldehydes. The method opens new strategic opportunities for the synthesis of chlorinated natural products, such as the chlorosulfolipids.

Second Generation of Aldol Reaction

Since the discovery of the Mukaiyama aldol reaction more than 40 years ago, several landmark publications have inspired researchers in the field. The Mukaiyama AR is one of the most significant named reactions in organic synthesis. In the past few decades, development of the modern AR has been at the forefront in addressing the challenges of regio-, chemo-, diastereo-, and enantioselectivity in organic synthesis. All of these selectivity challenges maybe present in a single pair of reactants, thus controlling the outcome of such a process has great practical value. More than 10 years ago, our group became involved in this iconic carbon-carbon bond-forming process and attempted to very closely investigate all possible features of the AR to solve several issues still encountered by chemists, most notably the selectivity challenges mentioned above. In this context, our group initiated the second generation of the AR based on a Lewis or Brønsted acid-catalyzed process in conjunction with the use of a "super silyl" (tris(trimethylsilyl)silyl) directing group, which has demonstrated unrivalled properties in controlling the outcome of the AR. Using the extraordinary power of the super silyl group, we were able to develop new methods and concepts that broadly impacted the ability to control the selectivity attributes and thus allowed for a highly stereoselective construction of polyketide, halogenated polyketide, polypropionate, and polyol scaffolds through inter- and/or intramolecular aldolization protocols. Our diastereoselective ARs of super silyl enol ethers and aldehydes have shown great efficiency and modularity in producing exclusively and preferentially syn- or anti-adducts, creating up to four new adjacent stereocenters in a one-pot sequential manner and under mild reaction conditions. The super silyl-directed AR does not only provide a solution to stereochemistry control challenges, but also offers an efficient, modular and high yielding technique toward nontrivial construction of complex architectures with unprecedented ease. We believe that the new Lewis- or Brønsted-acid-catalyzed super-silyl-directed AR processes chronicled in our laboratories have come to maturity and now offer a "road map" for strategic stereoselective synthesis of polyketide-like units. Herein we report our recent achievements in the diastereoselective C-C bond formation, through the super-silyl-directed AR, toward the synthesis of complex and sophisticated hydroxy aldehydes. We would like to note that due to the extremely broad range of work reported in this field, only stereoselective AR involving aldehyde-derived super SEEs will be discussed in this Account.

Synthesis of malhamensilipin A exploiting iterative epoxidation/chlorination: experimental and computational analysis of epoxide-derived chloronium ions

We report a 12-step catalytic enantioselective formal synthesis of malhamensilipin A (3) based upon an iterative epoxidation/chlorination strategy.

We report a 12-step catalytic enantioselective formal synthesis of malhamensilipin A (3) and diastereoisomeric analogues from (E)-2-undecenal. The convergent synthesis relied upon iterative epoxidation and phosphorus(v)-mediated deoxydichlorination reactions as well a titanium-mediated epoxide-opening to construct the C11–C16 stereohexad. The latter transformation occurred with very high levels of stereoretention regardless of the C13 configuration of the parent epoxide, implicating anchimeric assistance of either the γ- or δ-chlorine atoms, and the formation of chloretanium or chlorolanium ions, respectively. A computational analysis of the chloronium ion intermediates provided support for the involvement of chlorolanium ions, whereas the potential chloretanium ions were found to be less likely intermediates on the basis of their greater carbocationic character.

Triphosgene-pyridine mediated stereoselective chlorination of acyclic aliphatic 1,3-diols

We describe a strategy to chlorinate stereocomplementary acyclic aliphatic 1,3-diols using a mixture of triphosgene and pyridine. While 1,3-anti diols readily led to 1,3-anti dichlorides, 1,3-syn diols must be converted to 1,3-syn diol monosilylethers to access the corresponding 1,3-syn dichlorides. These dichlorination protocols were operationally simple, very mild, and readily tolerated by advanced synthetic intermediates.

Catalytic Enantioselective Dihalogenation and the Selective Synthesis of (-)-Deschloromytilipin A and (-)-Danicalipin A

A titanium-based catalytic enantioselective dichlorination of simple allylic alcohols is described. This dichlorination reaction provides stereoselective access to all common dichloroalcohol building blocks used in syntheses of chlorosulfolipid natural products. An enantioselective synthesis of ent-(-)-deschloromytilipin A and a concise, eight-step synthesis of ent-(-)-danicalipin A are executed and employ the dichlorination reaction as the first step. Extension of this system to enantioselective dibromination and its use in the synthesis of pentabromide stereoarrays relevant to bromosulfolipids is reported. The described dichlorination and dibromination reactions are capable of exerting diastereocontrol in complex settings allowing X-ray crystal structure analysis of natural and unnatural diastereomers of polyhalogenated stereohexads.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

The Alga Ochromonas danica Produces Bromosulfolipids

Many halogenases interchangeably incorporate chlorine and bromine into organic molecules. On the basis of an unsubstantiated report that the alga Ochromonas danica, a prodigious producer of chlorosulfolipids, was able to produce bromosulfolipids, we have investigated the promiscuity of its halogenases toward bromine incorporation. We have found that bromosulfolipids are produced with the exact positional and stereochemical selectivity as in the chlorosulfolipid danicalipin A when this alga is grown under modified conditions containing excess bromide ion.

Bioisosteric Exchange of Csp3 -Chloro and Methyl Substituents: Synthesis and Initial Biological Studies of Atpenin A5 Analogues

Asymmetric synthesis and initial biological studies of two analogues of a naturally occurring chlorinated antifungal agent, atpenin A5, are described. These analogues were selected on the basis of Cl→CH3 or H3 C→Cl exchanges in the side-chain of atpenin A5. The interchange of chloro and methyl substituents led to complex II inhibitors with equal IC50 values. This suggests that Cl↔Me bioisosteric exchange can be realized in aliphatic settings.

Synthesis and Biological Evaluation of Bromo- and Fluorodanicalipin A

We disclose the syntheses of (+)-bromodanicalipin A as well as (±)-fluorodanicalipin A. The relative configuration and ground-state conformation in solution of both molecules was secured by J-based configuration analysis which revealed that these are identical to natural danicalipin A. Furthermore, preliminary toxicological investigations suggest that the adverse effect of danicalipin A may be due to the lipophilicity of the halogens.

Biological Investigations of (+)-Danicalipin A Enabled Through Synthesis

A total synthesis of the chlorosulfolipid (+)-danicalipin A has been accomplished in 12 steps and 4.4% overall yield. The efficient and scalable synthesis enabled in-depth investigations of the lipid's biological properties, in particular cytotoxicity towards various mammalian cell lines. Furthermore, the ability of (+)-danicalipin A to increase the uptake of fluorophores into bacteria and mammalian cells was demonstrated, indicating it may enhance membrane permeability. By comparing (+)-danicalipin A with racemic 1,14-docosane disulfate, and the diol precursor of (+)-danicalipin A, we have shown that both chlorine and sulfate functionalities are necessary for biological activity.

Stereochemical studies of the opening of chloro vinyl epoxides: cyclic chloronium ions as intermediates

A systematic study of the opening of a collection of chlorinated vinyl epoxides is reported, which includes experiments that implicate both five- and four-membered chloronium ions as plausible intermediates in this type of epoxide opening reaction.

Catalytic chemo-, regio-, and enantioselective bromochlorination of allylic alcohols

Herein we describe a highly chemo-, regio-, and enantioselective bromochlorination reaction of allylic alcohols, employing readily available halogen sources and a simple Schiff base as the chiral catalyst. The application of this interhalogenation reaction to a variety of substrates, the rapid enantioselective synthesis of a bromochlorinated natural product, and preliminary extension of this chemistry to dibromination and dichlorination are reported.

Enantioselective divergent syntheses of several polyhalogenated Plocamium monoterpenes and evaluation of their selectivity for solid tumors

The family of polyhalogenated monoterpenes from Plocamium counts over a hundred known members. Using glyceraldehyde acetonide as a chiral-pool precursor, an enantioselective and divergent strategy was developed that provides a blueprint for the synthesis of many of the small yet complex acyclic members of this family. The broad applicability of this approach is demonstrated with the short, eight-step synthesis of four natural products and three analogues. These syntheses are the first of any members of the acyclic polyhalogenated Plocamium monoterpenes and permitted the evaluation of their selectivity against a range of tumor cell lines.

General approach to the synthesis of the chlorosulfolipids danicalipin A, mytilipin A, and malhamensilipin A in enantioenriched form

A second-generation synthesis of three structurally related chlorosulfolipids has been developed. Key advances include highly stereocontrolled additions to α,β-dichloroaldehydes, kinetic resolutions of complex chlorinated vinyl epoxide intermediates, and Z-selective alkene cross metatheses of cis-vinyl epoxides. This strategy facilitated the synthesis of enantioenriched danicalipin A, mytilipin A, and malhamensilipin A in nine, eight, and 11 steps, respectively.

Approaches to the chemical synthesis of the chlorosulfolipids

Since the initial discovery of the chlorosulfolipids in 1969, the chemical synthesis community largely ignored these compounds for nearly four decades, perhaps because they contain a high density of chlorine atoms, which suggested that these molecules and any projected synthetic intermediates might be unstable. Beginning in 2008, a sudden flurry of synthesis activity by several research groups, including our own, appeared in the literature. In this Account, we highlight our work from the last several years on the chemical synthesis of the chlorosulfolipids. Our work in this area began with attempts to stereoselectively generate the abundant dichloroalcohol functional group arrangements in these natural targets. In these early studies, we learned that many polychlorinated intermediates were far more stable than anticipated. We also developed a method for the diastereoselective dichlorination of allylic alcohol derivatives that permitted access to the syn,syn-dichloroalcohol stereotriad found in several chlorosulfolipids. Concurrently, we investigated an approach to mytilipin A that included multiple intermediates bearing aldehydes with β-leaving groups, but this route proved intractable. However, we leveraged what we had learned from this approach into our first success in this area: we synthesized danicalipin A via a route that introduced all of the polar functional groups using alkene oxidation reactions. By adapting this relatively general strategy, we completed an enantioselective synthesis of malhamensilipin A. This body of work also resulted in the full stereochemical elucidation of danicalipin A and the structural revision of malhamensilipin A. Finally, with the advent of Z-selective alkene cross metathesis, we developed a second-generation synthesis that featured this strategy in place of a poorly performing Wittig olefination that plagued our first approach. In addition to this new convergent step, we developed a reliable protocol for diastereoselective addition to highly sensitive α,β-dichloroaldehydes and a method for kinetic resolution of complex vinyl epoxides. Altogether, these advances led to a synthesis of enantioenriched mytilipin A in only eight steps. In the context of this work, we discovered a number of highly stereoselective reactions that might offer new, broadly applicable lessons in acyclic stereocontrol. Moreover, this research testifies to the stability of polychlorinated molecules and should inspire confidence in the use of aliphatic chlorides in other applications, including in discovery chemistry.

Synthesis of β-hydroxy-α-haloesters through super silyl ester directed syn-selective aldol reaction

Super silyl haloesters including chloro- and bromoacetate were synthesized and utilized for aldol reactions to give syn-β-hydroxy-α-haloacetates in good yields with high diastereoselectivities. β-Hydroxy-α-fluoroacetate was obtained by lithiation of super silyl bromofluoroacetate. Sequential Darzens reactions provided cis-glycidic esters in moderate yields.

Chlorine, an atom economical auxiliary for asymmetric aldol reactions

An auxiliary strategy has been developed for asymmetric reactions of aldehydes in which the auxiliary itself is not chiral, but a single chlorine atom introduced via organocatalytic α-chlorination. The stereodirecting influence of the chlorine atom is then exploited prior to its removal by radical reduction. This strategy is demonstrated in the synthesis of several aldols (92-99% ee) and the natural products (+)-dihydroyashabushiketol and (+)-solistatin.

A synthesis of the chlorosulfolipid mytilipin A via a longest linear sequence of seven steps

Magnificent seven: The chlorosulfolipid mytilipin A was synthesized in racemic form in seven steps and in enantioenriched form in eight steps. Key transformations include a highly diastereoselective bromoallylation of a sensitive α,β-dichloroaldehyde, a kinetic resolution of a vinyl epoxide, a convergent and highly Z-selective alkene cross-metathesis, and a chemoselective and diastereoselective dichlorination of a complex diene.

α-Haloaldehydes: versatile building blocks for natural product synthesis

The diastereoselective addition of organometallic reagents to α-chloroaldehydes was first reported in 1959 and occupies a historically significant role as the prototypical reaction for Cornforth's model of stereoinduction. Despite clear synthetic potential for these reagents, difficulties associated with producing enantiomerically enriched α-haloaldehydes limited their use in natural product synthesis through the latter half of the 20th century. In recent years, however, a variety of robust, organocatalytic processes have been reported that now provide direct access to optically enriched α-haloaldehydes and have motivated renewed interest in their use as building blocks for natural product synthesis. This Highlight summarizes the methods available for the enantioselective preparation of α-haloaldehydes and their stereoselective conversion into natural products.

β-Siloxy-α-haloketones through highly diastereoselective single and double mukaiyama aldol reactions

Double-action haloketones: A super silyl group enabled the first highly diastereoselective Mukaiyama aldol reactions of α-chloro- and α-fluoroketones with a wide range of aldehydes, providing anti-β-siloxy-α-haloketones. This process is compatible with one-pot double-aldol methodology and allows for rapid access to new halogen-modified polyketide fragments bearing up to four contiguous stereocenters.

Total synthesis and stereochemical revision of the chlorinated sesquiterpene (±)-gomerone c

Revised: the total synthesis of gomerone C results in revision of the stereochemical assignment at C3. The synthetic strategy relies on a late-stage Conia-ene reaction, which efficiently forms the bicyclo[3.2.1]octane containing the bridgehead chloride and generates an exocyclic olefin, which can be used as a flexible handle for further elaboration. The two contiguous quaternary centers are installed by means of a Diels-Alder reaction.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.